The invention relates to a method for calibrating an apparatus for the position measurement of measurement structures on a lithography mask, also referred to hereinafter as position measuring apparatus, a calibration mask for calibrating an apparatus of this type, and also a calibration mask set comprising calibration masks of this type. Furthermore, the invention relates to an arrangement comprising an apparatus for position measurement and also a calibration mask of this type, a use of a calibration mask of this type, and also a method for measuring a mask for microlithography.
The highly accurate position measurement of measurement structures, such as alignment marks, for instance, on a lithography mask is among the central tasks of mask metrology. It is also referred to as photomask pattern placement (PPPM). By means of the measurement of the measurement structures, the material measure on the mask is generated with a high accuracy. It is an essential prerequisite for enabling the positional accuracy of the structures on the mask at all in the mask writing process using electron beam writers. Furthermore, the measurement of the measurement structures of an existing mask set makes it possible to qualify the deviation of the structure positions of the different masks for the individual lithographic layers with respect to one another. This deviation of the structure positions from mask to mask is also referred to as “overlay”. Masks in the sense mentioned above are often also referred to as reticles.
As the mask structures shrink from technology node to technology node, the requirements made of the position measurement of the mask structures also increase continuously. As a result of technologies such as double patterning, the requirements made of the mask-to-mask overlay and thus of the structure positioning increase significantly in addition to that. Since the individual masks of a mask set are increasingly being produced by different mask manufacturing firms, often spread throughout the world, and measured by means of different position measuring apparatuses, also referred to as “registration apparatuses”, the coordination of the individual position measuring apparatuses with respect to one another is acquiring ever increasing importance.
The position determination on lithography masks is conventionally based exclusively on an interferometric length measurement. For this purpose, alignment marks of a mask are detected individually with regard to their position by means of a microscopic image. By means of a positioning table, the individual alignment marks of the mask are successively moved into the center of the image field and the position of the respective alignment mark is determined by means of edge threshold values or by means of correlation methods. The distance from the previously measured alignment mark is thereupon determined by determining the distance covered by the positioning table between the measurements. The distance covered by the positioning table is determined by means of interferometric length measurement.
The calibration of position measuring apparatuses is conventionally effected by means of self-consistency tests. In this case, a calibration mask is measured in different insertion positions and rotational positions. From the quasi-redundant measurement data record, position errors of the alignment marks on the calibration mask can be separated from inherent errors of the position measuring apparatus. The latter are used in turn for calibrating the position measuring apparatus.
Typical causes of errors of the position measuring apparatus are, inter alia, interferometer errors and also tilting and unevennesses of the interferometer mirrors. Although such errors can be taken into account by calibration methods described above, such methods nevertheless remain bound to the measurement at the position measuring apparatus itself. This leads, in particular, to the problems presented below.
Each calibration method on the abovementioned basis is blind to specific types of error intrinsic to it. Thus, specific classes of errors cannot be detected and separated by a simple calibration measurement. Examples of the causes of such errors are, inter alia: mirror unevennesses with spatial frequencies greater than the calibration raster resulting from the different insertion positions, incorrect positions of the mask, image field rotation, unevennesses of the mask, etc.
This problem is conventionally combated by increasing the redundancy of the measurements. However, this significantly increases the measurement outlay. The measurement outlay for the calibration thus increases with the accuracy requirements and the calibration quality.
By matching the individual position measuring apparatuses of the same type to one another, failures of individual machines can be registered. Systematic errors which are inherent to the method and inherent to the machine type are not identified, however.